The new, unified Sylius API is still under development.
We have decided that we should rebuild our API and use API Platform to build a truly mature, multi-purpose API which can define a new standard for headless e-commerce backends.
We will be supporting API Platform out-of-the-box. Secondly, it means that both APIs (Admin API and Shop API) will be deprecated. We are not dropping them right now, but they will not receive further development. In the later phase, we should provide an upgrade path for currently working apps. Last, but not least, you can already track our progress. All the PR’s will be aggregated in this issue and the documentation can be already found here.